1. Field of Invention
This invention generally relates to characterization of samples, particularly to optical systems for characterizing samples such as patterned and unpatterned semiconductor wafers.
2. Related Art
Optical techniques may be used to obtain information about materials. For example, optical techniques may be used to characterize substrates such as semiconductor wafers.
As the device density on wafers increases, it is more important to quickly obtain accurate information about the unpatterned (blanket) and patterned substrates. However, existing techniques may be time-consuming and cumbersome, and may not sample the wafer adequately. Additionally, some existing techniques are destructive; that is, they require that the wafer be damaged in order to analyze the patterned device elements. Therefore, characterization of actual product wafers may not be performed.
One technique that may be used to characterize patterned wafers is the inspection of patterns using a high magnification optical microscope, scanning electron microscope (SEM), or other imaging technique. However, these techniques may not provide a complete picture of the wafer patterns. Since a patterned wafer may contain millions or tens of millions of device elements (e.g., transistors), only a small percentage of the device elements may be characterized.
Another technique that may be used to characterize wafers is ellipsometry. Ellipsometry is an optical technique that measures the change in polarization as light is reflected off a surface. Although ellipsometry is an important tool for obtaining information about some sample characteristics (e.g., for measuring layer thickness and refractive index), it does not provide information about some other sample characteristics, such as stress and pattern integrity.